MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OP DORIS. 211 
are formed and developed the spiny teeth in regular rank and file ; the upper third 
contains the mature teeth, which can all more or less readily be used in prehension. 
Development of Teeth . — If we make a transverse section of that lower part of the 
tube* which projects obliquely behind from the buccal mass, and examine it, we find 
in the centre, the cuneiform lamina having a somewhat pyriform shape, the base for- 
wards, the apex backwards and attached to the muscular envelope of the pouch. 
Except at this point the lamina is free and lies in a cavity bounded by two parallel 
membranes, between which are developed the rows of teeth ; the outer of these is the 
dentigerous layer, which rests upon the mucous membrane lining the muscular enve- 
lope of the tube. If we make another section-^ lower down, and obliquely through 
the end of the pouch, we find the cuneiform lamina fixed at both ends, the free edge 
has ceased, and the lamina is found attached along the median line of the fundus of 
the tube. Along each side of this attachment is found an enlargement of the mucous 
membrane, which must be of glandular nature, as from it seem to originate the two 
parallel membranes mentioned in noticing the former section just above. 
Between these membranes, as already stated, the teeth are formed. These organs 
can be observed at a very early period of their growth. They may be discerned upon 
the newly-secreted edge of the outer or dentigerous layer as delicate rows of elongated 
and attenuated, soft and colourless cells. At the distance of a row or two further 
on they assume the exact form and size of the mature teeth, still, however, retaining 
their soft cell-like character. The next rows seem orange-coloured, and have put on 
some of the hardness and of the other characters of perfect teeth, which are however 
somewhat paler. The teeth examined just below the transverse membrane that 
closes in the teeth follicle above, are found to have attained all that extreme hardness 
and temper which characterize them when in use, and possess the most perfect peilu- 
cidity and polish. 
The space between the two parallel membranes is packed with a soft matter which 
is derived from the inner layer, and which, when placed in spirit, becomes flocculent ; 
in this matter the teeth are imbedded. These two membranes may not inaptly be 
compared to the pulp and enamel membranes of the teeth of the Vertebrata, though 
the denticles themselves, as far as we have been able to observe, are quite homoge- 
neous in structure. We are inclined to think the denticles are siliceous in composi- 
tion, as they resist the action of the stronger mineral acids, as do those of EoUs, 
which, although they possess the same hardness, transparency and polish, are in 
some measure affected by hydrofluoric acid. 
Those teeth which are placed upon the top of the lingual mass, and are actually 
engaged in the act of prehension, become gradually blunted, broken or wholly de- 
tached, and that the more rapidly the more they lie away from the median line : 
hence the whole apparatus is worn to a point in front and on the median line. This 
continual wear and tear of the dental rows necessitates a fresh supply of them, and 
this is constantly being provided by the secretion of dentigerous membrane at the 
* Plate XIV. figs. 1 and 3. f Plate XIV. fig. 2. 
